Neutralization Process

ABSTRACT

The invention relates to a continuous neutralization process in which at least one ethylenically unsaturated carboxylic acid is neutralized at least partly with a base and the temperature of the neutralized solution is less than 70° C., and also to an apparatus for carrying out the process.

The present invention relates to a continuous neutralization process forethylenically unsaturated carboxylic acids and to an apparatus forcarrying out the process.

Further embodiments of the present invention can be taken from theclaims, the description and the examples. It is evident that thefeatures of the inventive subject matter which have been mentioned aboveand are yet to be explained below are usable not only in the combinationspecified in each case but also in other combinations without leavingthe scope of the invention.

Water-absorbing polymers are especially polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable graft base, crosslinked cellulose ethers orstarch ethers, crosslinked carboxymethylcellulose, partly crosslinkedpolyalkylene oxide or natural products swellable in aqueous liquids, forexample guar derivatives, preference being given to water-absorbingpolymers based on partly neutralized acrylic acid. Such polymers areused as products that absorb aqueous solutions to produce diapers,tampons, sanitary napkins and other hygiene articles, but also aswater-retaining agents in market gardening.

The preparation of the water-absorbing polymers is described, forexample, in the monograph “Modern Superabsorbent Polymer Technology”, F.L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann'sEncyclopedia of Industrial Chemistry, 6th Edition, Volume 35, pages 73to 103. The preferred preparation process is solution or gelpolymerization. In this technology, a monomer mixture is firstlyprepared and is neutralized batchwise and then transferred to apolymerization reactor, or initially charged actually within thepolymerization reactor. In the batchwise or continuous process whichfollows, the reaction is effected to give the polymer gel which, in thecase of a stirred polymerization, is already in comminuted form. Thepolymer gel is subsequently dried, ground and sieved and thentransferred to further surface treatment.

A continuous polymerization process forms the basis, for example, of WO01/38402, in which the aqueous monomer solution is fed continuously to amixing kneader with at least two axially parallel-rotating shafts.

Continuous gel polymerizations are also known from WO 03/004237, WO03/022896 and WO 01/016197.

Both in the continuous and in the batchwise polymerization, the acrylicacid is neutralized batchwise in the case of preneutralization.Typically, the reactants (acrylic acid, water, optional comonomers andsodium hydroxide solution) are metered in and mixed batchwise in thepolymerization reactor in the case of solution polymerization. In thisstep, the remaining course of the polymerization and also the expectedpolymer properties are laid down to a very substantial extent. Thedegree of crosslinking of the base polymer and the degree ofneutralization are typically determined in this step. The degree ofneutralization of the monomers is between 0 and 80 mol %. In the case ofacidic polymerization, the resulting polymer gel is typicallyneutralized afterward to an extent of from 50 to 80 mol %, preferably toan extent of from 60 to 75 mol %, by adding sodium hydroxide or sodiumcarbonate solution to the acidic polymer gel and incorporating it.

Neutralization processes are described, for example, in EP-A 0 372 706,EP-A 0 574 260, WO 03/051415 and EP-A 1 470 905.

EP-A 0 372 706 describes a three-stage neutralization process in whichacrylic acid and sodium hydroxide solution are metered in simultaneouslyin a first stage in such a way that a degree of neutralization of from75 to 100 mol % is maintained, the degree of neutralization is raised tofrom 100.1 to 110 mol % in a second stage in order to hydrolyzediacrylic acid present as an impurity in the acrylic acid used, and adegree of neutralization of from 20 to 100 mol % is established in athird stage by addition of further acrylic acid.

EP-A 0 574 260 discloses, on page 7, lines 38 to 41, that sodiumhydroxide solution is advantageously initially charged in theneutralization and acrylic acid is subsequently added with cooling.

WO 03/051415 teaches a process for preparing water-absorbing polymers,in which the monomer solution has a minimum temperature of 40° C.

EP-A 1 470 905 describes, in the examples, the continuous neutralizationof acrylic acid immediately upstream of the polymerization reactor.Owing to the heat of neutralization, the temperature rises to 95° C.

It is known that the reactivity of acrylic acid differs very greatlyfrom that of its salts, which is why the course of the polymerization isalso greatly dependent upon the pH at which it takes place. In a graphicillustration in the monograph “Modern Superabsorbent PolymerTechnology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or inUllmann's Encyclopedia of Industrial Chemistry, 6th Edition, Volume 35,page 35, the polymerization rate is plotted as a function of the pH.According to this, the polymerization rate passes through a minimum at apH of from 6 to 7. However, this corresponds to the pH which isgenerally desired in the saleable products. This behavior is explainedby the occurrence of electrostatic repulsion reactions, which are notpresent in the case of very substantially undissociated acrylic acid,between the charged monomers in salt form and the growing free-radicalchain, which leads to slowing of the reaction.

It is also known that unneutralized acrylic acid can be polymerized moreeasily than preneutralized systems. However, this difference is reducedin the case of rising monomer concentration, in particular because ahigher monomer concentration suppresses the dissociation of the acrylicacid salts.

In order to take account of all of these details of the reactionmechanism, compromises are typically entered into in conducting thereaction.

Generally, the degree of neutralization of the acrylic acid isestablished actually before it enters the continuous polymerization. Theneutralization is effected batchwise. Batchwise neutralization has theadvantage that acrylic acid and/or sodium hydroxide solution can bemetered in under temperature control. This prevents overheating andundesired polymerization in the mixture vessel. The degree ofneutralization is selected in accordance with the polymerizationconditions and the desired absorption profile and, if desired, correctedin a subsequent neutralization which is usually effected on the polymergel.

A particular disadvantage of neutralization as a preceding batchwiseprocess step is the logistical demands. The provision of reservoirtanks, vessels or containers which store the partly neutralized acrylicacid, and also the complete apparatus demands have a disadvantageouseffect on the economic viability of the process.

It would therefore be desirable to carry out the neutralizationcontinuously. Continuous neutralization could then be combinedadvantageously with a continuous polymerization, in which case thelogistical demands and the provision of reservoir vessels is minimized.However, continuous neutralization is found to be problematic owing tothe evolution of heat which occurs, since this results in undesiredpremature polymerization. In all cases, the further course of thecontinuous polymerization is adversely effected by polymer gel which hasalready formed, especially as a result of blockages in lines which haveonly been designed for liquid transport.

It was an object of the present invention to provide a continuousneutralization process in which the occurrence of undesirably hightemperature peaks can be avoided.

The object is achieved by a neutralization process in which at least oneethylenically unsaturated carboxylic acid is neutralized at least partlywith a base, which comprises carrying out the neutralizationcontinuously and the temperature of the neutralized solution being lessthan 70° C.

Preference is given to using ethylenically unsaturated carboxylic acidssuch as acrylic acid, methacrylic acid, maleic acid, fumaric acid anditaconic acid. Acrylic acid is particularly preferred.

The temperature of the ethylenically unsaturated carboxylic acid istypically from 0 to 40° C., preferably from 5 to 35° C., more preferablyfrom 10 to 30° C., most preferably from 15 to 25° C., while ensuringsufficient distance from melting point. In the case of use of acrylicacid, the temperature should not go below 15° C. in any case.

A preferred base is aqueous alkali. Aqueous alkali is all aqueoussolutions with an alkaline reaction, i.e. aqueous solutions with a pH ofat least 8, preferably at least 10, more preferably at least 12, mostpreferably at least 14.

The alkaline salts usable in the aqueous neutralizing agent arepreferably alkali metal hydroxides, alkali metal oxides, alkali metalcarbonates and alkali metal hydrogencarbonates and mixtures thereof.Instead of alkali metal salts, it is also possible to use ammoniumsalts. Sodium and potassium are particularly preferred as alkali metals,but very particular preference is given to sodium hydroxide, sodiumcarbonate or sodium hydrogencarbonate and mixtures thereof. Typically,the alkali content in the aqueous alkali is at least 10% by weight,preferably at least 20% by weight, more preferably at least 30% byweight, most preferably at least 40% by weight.

The temperature of the aqueous alkali is typically from 0 to 45° C.,preferably from 5 to 40° C., more preferably from 10 to 35° C., mostpreferably from 15 to 30° C., while avoiding oversaturations and thusprecipitations.

When the alkali content of the aqueous alkali is at least 25% by weight,higher temperatures are advantageous, typically of from 10 to 60° C.,preferably of from 20 to 55° C., more preferably of from 30 to 50° C.,most preferably of from 40 to 45° C.

The ratio of ethylenically unsaturated carboxylic acid to base istypically selected such that the degree of neutralization of theethylenically unsaturated carboxylic acid, after neutralization, ispreferably from 25 to 85 mol %, preferentially from 27 to 80 mol %, morepreferably from 27 to 30 mol % or from 40 to 75 mol %.

The degree of neutralization is the molar ratio of neutralizedethylenically unsaturated carboxylic acid after neutralization to thetotal amount of ethylenically unsaturated carboxylic acid used beforeneutralization.

The neutralization is carried out continuously. This means thatethylenically unsaturated carboxylic acid and/or base are supplied tothe neutralization region and neutralized solution is simultaneouslywithdrawn from the neutralization region. Of course, startup andshutdown operations of the continuous neutralization process areexcluded from this.

The solution withdrawn continuously from the neutralization region is atleast partly transferred continuously into the polymerization.

The polymerization is preferably likewise carried out continuously.

The neutralization region is the region in which the neutralizationtakes place to a substantial extent, i.e. the region in whichethylenically unsaturated acid and base react with salt formation(neutralization).

The neutralization has substantially been completed when the conversionof the neutralization is at least 90 mol %, preferably at least 95 mol%, more preferably at least 98 mol %, most preferably at least 99 mol %.The conversion can be determined easily via the heat of neutralizationreleased by comparison with the theoretical exothermicity.

The continuous neutralization is carried out in such a way that thetemperature of the neutralized solution is preferably less than 60° C.,preferentially less than 50° C., more preferably less than 40° C., mostpreferably less than 30° C., the temperature being the averagetemperature after neutralization, i.e. the mean temperature after fullexothermicity.

The neutralization process according to the invention is of course alsosuitable for preparing neutralized solutions with a temperature of 70°C. and more. However, the polymerization tendency of the solution riseswith rising temperature.

The distance between neutralization and polymerization is typically atleast 1 m, preferably at least 5 m, more preferably at least 10 m, mostpreferably at least 20 m, and typically not more than 100 m, thedistance being the length of the route that the monomer solution passesthrough in passing directly between the alkali metering andpolymerization reactor.

In addition, the neutralized solution may be diluted with water. Thedilution with water allows the solids content of the neutralizedsolution to be adjusted. The solids content is the sum of theproportions by weight of neutralized ethylenically unsaturatedcarboxylic acid and, if desired, excess ethylenically unsaturatedcarboxylic acid or excess base. The solids content of the neutralizedsolution is typically from 10 to 80% by weight, preferably from 20 to70% by weight, more preferably from 30 to 60% by weight.

The temperature of the water is typically from above 0 to 40° C.,preferably from 5 to 35° C., more preferably from 10 to 30° C., mostpreferably from 15 to 25° C.

Advantageously, the neutralized solution is cooled, in which case theheat exchangers usable for cooling are not subject to any restriction.The neutralized solution is cooled to a temperature of preferably lessthan 50° C., preferentially less than 40° C., more preferably less than30° C., most preferably less than 20° C. The cooling should be as closeas possible to the neutralization, since high residence times of theneutralized solution at high temperatures can thus be avoided.

Preference is given to premixing water and base. In this case, the heatof dissolution released can be removed actually before theneutralization, for example by means of suitable heat exchangers.

In a particularly preferred embodiment of the present invention, aportion of the neutralized solution is recycled into the neutralization,preferably cooled.

The recycling allows the heat of neutralization and the heat ofdissolution to be distributed better and temperature peaks (peaktemperature) in the mixture to be kept low. The proportion of recycledneutralized solution is typically from 25 to 99%, preferably from 33 to98%, more preferably from 50 to 95%, most preferably from 80 to 90%,based in each case on the neutralized solution.

The ethylenically unsaturated carboxylic acid, the base and, if desired,the water may be metered into the recycled neutralized solution at anypoint. Preference is given to metering in the liquids in succession,particular preference to metering in base and ethylenically unsaturatedcarboxylic acid in succession, very particular preference to metering inwater, base and ethylenically unsaturated carboxylic acid in succession.

Advantageously, at least one of the reactants is metered in via two ormore separate addition points.

For example, the reactants may be metered in via two, three, four, fiveor six addition points, the addition points preferably being arrangedsuch that they have a common axis (for two addition points) or form asymmetrical star (for at least three addition points), and the axis orstar is at right angles to the flow direction of the neutralizedsolution (multiple addition points).

The base is metered in particularly advantageously when two, three orfour multiple addition points are arranged in succession.

The division into a plurality of addition points brings about moreuniform mixing and lower temperature peaks, which reduces the risk ofundesired polymerization.

In a further embodiment, water and base are metered in such that thewater encloses the base on entry into the neutralization. To this end,for example, two tubes inserted into one another may be used, in whichcase the base is metered in through the inner tube and the water throughthe annular gap between inner and outer tube.

Advantageously, the neutralization comprises an additional vessel as abuffer vessel.

An exemplary inventive neutralization is shown by FIG. 1, the referencesymbols having the following definitions:

Z₁ to Z₃ feeds for reactants 1 to 3 A outlet B vessel P pump R ring lineW heat exchanger

By means of a pump P, neutralized solution is recycled partly via thering line R. The rest of the neutralized solution is sent via the outletA to further use. The vessel B serves as a buffer. 50% by weight sodiumhydroxide solution is preferably metered in via inlet Z₁, preferablyacrylic acid via inlet Z₂ and preferably water via inlet Z₃.

In order that the reactants are mixed very intensively into the recycledneutralized solution, the flow at the point of mixing-in should be veryturbulent. The mixing-in point is the place where the particularreactant meets the recycled neutralized solution.

In a preferred embodiment of the present invention, at least one of thereactants is metered into a Venturi tube; preferably, all reactants aremetered into a Venturi tube; more preferably, all reactants are meteredinto a common Venturi tube.

A Venturi tube is a pipe constriction of a restricted length in whichpressure drop is converted substantially reversibly to kinetic energy.To this end, the cross section F₁ is reduced to the cross section F₂over the zone L₁, the cross section F₂ is kept constant over the zone L₂and the cross section F₂ is widened again to the cross section F₁ overthe zone L₃. The cross section F₁ is greater than the cross section F₂and the length L₃ is greater than the length L₁.

The reactants for the neutralization are preferably metered in in theregion of the zone L₂ with the cross section F₂.

The optimal design of a Venturi tube is known per se to those skilled inthe art. The Venturi tube is preferably designed such that the pressurein the region of the zone L₂ is less than the ambient pressure (suctionconveying) and/or that the flow in the region of the zone L₂ isturbulent, in which case the Reynolds number should be at least 1000,preferably at least 2000, more preferably at least 3000, most preferablyat least 4000, and typically less than 10 000 000.

The inventive continuous neutralization process enablespolymerization-sensitive ethylenically unsaturated acids to beneutralized in a particularly gentle manner. Local overheating which caninduce undesired polymerization is reliably prevented.

The continuous neutralization process according to the invention is alsosuitable for batchwise polymerization processes. For example, the vesselB according to FIG. 1 can be used as a reservoir vessel for a batchwisepolymerization. Compared to the conventional batchwise neutralization,the continuous neutralization process enables distinctly improved heatremoval.

The present invention further provides a process for preparingwater-absorbing polymers by using a neutralized solution prepared by theneutralization process according to the invention as the monomersolution.

Preference is given to combining the inventive continuous neutralizationprocess with a continuous polymerization process, in which casepreference is given to carrying out all process steps, such asneutralization, polymerization, drying, grinding, sieving,postcrosslinking and sieving, continuously.

The water-absorbing polymers are obtained, for example, bypolymerization of a monomer solution comprising

-   a) at least one ethylenically unsaturated carboxylic acid,-   b) at least one crosslinker,-   c) if desired one or more ethylenically and/or allylically    unsaturated monomers copolymerizable with the monomer a), and-   d) if desired one or more water-soluble polymers onto which the    monomers a), b) and if appropriate c) can be at least partly    grafted.

Suitable ethylenically unsaturated carboxylic acids a) are, for example,acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol refers to compounds of the following formula:

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl and R⁴ is hydrogen or an acyl radical having from 1 to 20carbon atoms.

Preferred R⁴ radicals are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically tolerable carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹=R²=R³=methyl,especially racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. Especially preferred is RRR-alpha-tocopherol.

The monomer solution comprises preferably not more than 130 ppm byweight, more preferably not more than 70 ppm by weight, preferably notless than 10 ppm by weight, more preferably not less than 30 ppm byweight and especially about 50 ppm by weight of hydroquinone monoether,based in each case on acrylic acid, with acrylic acid salts beingcounted as acrylic acid. For example, the monomer solution can beprepared using acrylic acid having an appropriate hydroquinone monoethercontent.

The crosslinkers b) are compounds having at least two polymerizablegroups which can be free-radically polymerized into the polymer network.Suitable crosslinkers b) are, for example, ethylene glycoldimethacrylate, diethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, asdescribed in EP-A-0 530 438, di- and triacrylates, as described in EP-A0 547 847, EP-A 0 559 476, EP-A 0 632 068, WO 93/21237, WO 03/104299, WO03/104300, WO 03/104301 and DE-A 103 31 450, mixed acrylates which, aswell as acrylate groups, comprise further ethylenically unsaturatedgroups, as described in DE-A 103 31 456 and WO 04/013064, or crosslinkermixtures as described, for example, in DE-A 195 43 368, DE-A 196 46 484,WO 90/15830 and WO 02/32962.

Suitable crosslinkers b) include in particularN,N′-methylenebisacrylamide and N,N′-methylenebismethacrylamide, estersof unsaturated mono- or polycarboxylic acids of polyols, such asdiacrylate or triacrylate, for example butanediol diacrylate, butanedioldimethacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate and also trimethylolpropane triacrylate and allylcompounds, such as allyl (meth)acrylate, triallyl cyanurate, diallylmaleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid and alsovinylphosphonic acid derivatives as described, for example, in EP-A 0343 427. Suitable crosslinkers b) further include pentaerythritoldiallyl ether, pentaerythritol triallyl ether, pentaerythritoltetraallyl ether, polyethylene glycol diallyl ether, ethylene glycoldiallyl ether, glycerol diallyl ether, glycerol triallyl ether,polyallyl ethers based on sorbitol, and also ethoxylated variantsthereof. In the process of the invention, it is possible to usedi(meth)acrylates of polyethylene glycols, the polyethylene glycol usedhaving a molecular weight between 300 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 15-tuply ethoxylated glycerol, of 3- to 15-tuplyethoxylated trimethylolpropane, of 3- to 15-tuply ethoxylatedtrimethylolethane, especially di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol, of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol, of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuplyethoxylated trimethylolpropane, of 40-tuply ethoxylated glycerol, of40-tuply ethoxylated trimethylolethane and also of 40-tuply ethoxylatedtrimethylolpropane.

Very particularly preferred crosslinkers b) are polyethoxylated and/or-propoxylated glycerols which have been esterified with acrylic acid ormethacrylic acid to di- or triacrylates, as described, for example, inWO 03/104301. Di- and/or triacrylates of 3- to 10-tuply ethoxylatedglycerol are particularly advantageous. Very particular preference isgiven to di- or triacrylates of 1- to 5-tuply ethoxylated and/orpropoxylated glycerol. The triacrylates of 3- to 5-tuply ethoxylatedand/or propoxylated glycerol are most preferred. These are notable forparticularly low residual levels (typically below 10 ppm by weight) inthe water-absorbing polymer and the aqueous extracts of thewater-absorbing polymers produced therewith have an almost unchangedsurface tension (typically not less than 0.068 N/m) compared with waterat the same temperature.

The amount of crosslinker b) is preferably from 0.01 to 1% by weight,more preferably from 0.05 to 0.5% by weight, most preferably from 0.1 to0.3% by weight, based in each case on the monomer a).

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the monomers a) are acrylamide, methacrylamide,crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Typically, the monomer solutions are substantiallyfreed of oxygen before the polymerization (inertization), for example bymeans of flowing an inert gas, preferably nitrogen, through them. Thisdistinctly weakens the action of the polymerization inhibitors. Theoxygen content of the monomer solution is preferably lowered to lessthan 1 ppm by weight and more preferably to less than 0.5 ppm by weightbefore the polymerization.

The preparation of a suitable base polymer and also further suitablehydrophilic ethylenically unsaturated monomers d) is described in DE-A199 41 423, EP-A 0 686 650, WO 01/45758 and WO 03/104300.

Water-absorbing polymers are typically obtained by additionpolymerization of an aqueous monomer solution and, if desired,subsequent comminution of the hydrogel. Suitable preparation methods aredescribed in the literature. Water-absorbing polymers are obtainable,for example, by

-   -   gel polymerization in a batch process or tubular reactor and        subsequent comminution in a meat grinder, extruder or kneader        (EP-A-0 445 619, DE-A-198 46 413)    -   addition polymerization in a kneader with continuous comminution        by contrarotatory stirring shafts for example (WO 01/38402)    -   addition polymerization on a belt and subsequent comminution in        a meat grinder, extruder or kneader (DE-A-38 25 366, U.S. Pat.        No. 6,241,928)    -   emulsion polymerization, which produces bead polymers having a        relatively narrow gel size distribution (EP-A-0 457 660)    -   in situ addition polymerization of a woven fabric layer which,        usually in a continuous operation, has previously been sprayed        with aqueous monomer solution and subsequently been subjected to        a photopolymerization (WO 02/94328, WO 02/94329).

The reaction is preferably carried out in a kneader as described forexample in WO 01/38402, or on a belt reactor as described for example inEP-A 0 955 086.

Neutralization can also be carried out partly after the polymerization,at the hydrogel stage. It is therefore possible to neutralize up to 40mol %, preferably from 10 to 30 mol % and more preferably from 15 to 25mol % of the acid groups before the polymerization by adding a portionof the neutralizing agent to the monomer solution and setting thedesired final degree of neutralization only after the polymerization, atthe hydrogel stage. The monomer solution can be neutralized by mixing inthe neutralizing agent. The hydrogel may be comminuted mechanically, forexample by means of a meat grinder, in which case the neutralizing agentcan be sprayed, sprinkled or poured on and then carefully mixed in. Tothis end, the gel mass obtained can be repeatedly ground in the meatgrinder for homogenization. Neutralization of the monomer solution tothe final degree of neutralization is preferred. The neutralizedhydrogel is then dried with a belt or drum dryer until the residualmoisture content is preferably below 15% by weight and especially below10% by weight, the water content being determined by EDANA (EuropeanDisposables and Nonwovens Association) recommended test method No.430.2-02 “Moisture content”. If desired, drying can also be carried outusing a fluidized bed dryer or a heated plowshare mixer. To obtainparticularly white products, it is advantageous to dry this gel whileensuring rapid removal of the evaporating water. To this end, the dryertemperature must be optimized, the air feed and removal has to becontrolled, and sufficient venting must be ensured in each case. Thehigher the solids context of the gel, the simpler the drying, by itsnature, and the whiter the product. The solids content of the gel beforethe drying is therefore preferably between 30% and 80% by weight. It isparticularly advantageous to vent the dryer with nitrogen or anothernonoxidizing inert gas. If desired, however, it is possible simply justto lower the partial pressure of the oxygen during the drying in orderto prevent oxidative yellowing processes. In general, though, adequateventing and removal of the water vapor also still lead to an acceptableproduct. A very short drying time is generally advantageous with regardto color and product quality.

The dried hydrogel is preferably ground and sieved, useful grindingapparatus typically including roll mills, pin mills or swing mills. Theparticle size of the sieved, dry hydrogel is preferably below 1000 μm,more preferably below 900 μm and most preferably below 800 μm, andpreferably above 100 μm, more preferably above 150 μm and mostpreferably above 200 μm.

Very particular preference is given to a particle size (sieve cut) offrom 106 to 850 μm. The particle size is determined according to EDANA(European Disposables and Nonwovens Association) recommended test methodNo. 420.2-02 “Particle size distribution”.

The base polymers are then preferably surface postcrosslinked.Postcrosslinkers suitable for this purpose are compounds comprising twoor more groups capable of forming covalent bonds with the carboxylategroups of the hydrogel. Suitable compounds are, for example, alkoxysilylcompounds, polyaziridines, polyamines, polyamidoamines, di- orpolyglycidyl compounds, as described in EP-A 0 083 022, EP-A 0 543 303and EP-A 0 937 736, di- or polyfunctional alcohols, as described in DE-C33 14 019, DE-C 35 23 617 and EP-A 0 450 922, or p-hydroxyalkylamides,as described in DE-A 102 04 938 and U.S. Pat. No. 6,239,230.

In addition, DE-A 40 20 780 describes cyclic carbonates, DE-A 198 07 5022-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone,DE-A 198 07 992 bis- and poly-2-oxazolidinones, DE-A 198 54 5732-oxotetrahydro-1,3-oxazine and its derivatives, DE-A 198 54 574N-acyl-2-oxazolidones, DE-A 102 04 937 cyclic ureas, DE-A 103 34 584bicyclic amide acetals, EP-A 1 199 327 oxetanes and cyclic ureas and WO03/031482 morpholine-2,3-dione and its derivatives, as suitable surfacepostcrosslinkers.

The postcrosslinking is typically carried out in such a way that asolution of the surface postcrosslinker is sprayed onto the hydrogel oronto the dry base polymer powder. After the spraying, the polymer powderis dried thermally, and the crosslinking reaction may take place eitherbefore or during drying.

The spraying with a solution of the crosslinker is preferably carriedout in mixers having moving mixing implements, such as screw mixers,paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to vertical mixers and very particularpreference to plowshare mixers and shovel mixers. Suitable mixers are,for example, Lodige® mixers, Bepex® mixers, Nauta® mixers, Processall®mixers and Schugi® mixers.

The thermal drying is preferably carried out in contact dryers, morepreferably shovel dryers and most preferably disk dryers. Suitabledryers are, for example, Bepex® dryers and Nara® dryers. It is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. It is equally possible to use a downstream dryer,for example a tray dryer, a rotary tube oven or a heatable screw. It isalso possible, for example, to utilize an azeotropic distillation as adrying process.

Preferred drying temperatures are in the range from 50 to 250° C.,preferably in the range from 50 to 200° C. and more preferably in therange from 50 to 150° C. The preferred residence time at thistemperature in the reaction mixer or dryer is below 30 minutes and morepreferably below 10 minutes.

The present invention further provides an apparatus for carrying out thecontinuous neutralization process according to the invention, comprising

-   i) a ring line R,-   ii) at least one first inlet Z₁ into the ring line R, iii) at least    one second inlet Z₂ into the ring line R,-   iv) at least one heat exchanger W in the ring line R, the heat    exchanger W being disposed beyond the inlets Z₁ and Z₂ in flow    direction,-   v) at least one outlet A from the ring line R, the outlet A being    disposed beyond the heat exchanger W in flow direction,-   vi) a pump P and-   vii) if desired, a vessel B between the heat exchanger W and the    outlet A,    where at least one first inlet Z₁ means that reactant 1, for example    sodium hydroxide solution, is supplied via one or more inlets Z₁,    and at least one second inlet Z₂ means that reactant 2, for example    acrylic acid, is supplied via one or more inlets Z₂.

The ring line cross section Q is preferably from 20 to 2000 cm², morepreferably from 80 to 700 cm², most preferably from 200 to 500 cm². Thering line R preferably has a circular cross section.

The totality of the inlets Z₁ has a cross section of preferably from 1.5to 100 cm², more preferably from 6 to 35 cm², most preferably from 15 to25 cm². The inlets Z₁ preferably have a circular cross section.

The totality of the inlets Z₂ has a cross section of preferably from 1.5to 100 cm², more preferably from 6 to 35 cm², most preferably from 15 to25 cm². The inlets Z₂ preferably have a circular cross section.

The pump P has a delivery capacity of preferably from 1 to 1000 t/h,more preferably from 10 to 700 t/h, most preferably from 100 to 500 t/h.

The vessel B has a volume of preferably from 1 to 100 m³, morepreferably from 10 to 100 m³, most preferably from 20 to 50 m³.

The inlets Z₁ and Z₂ are preferably arranged in succession, the inletsZ₁ preferably being before the inlets Z₂ in flow direction.

The distance between the inlets Z₁ and Z₂ is preferably from 10 to 500%,more preferably from 50 to 300%, most preferably from 80 to 200%, of thesquare root of the ring line cross section Q.

Preferably at least two inlets Z₁ and/or Z₂ are present, more preferablytwo, three, four, five or six inlets Z₁ and Z₂, the inlets Z₁ and Z₂preferably being arranged such that they have a common axis (for twoinlets Z₁ and Z₂) or form a symmetrical star (for at least three inletsZ₁ and Z₂) and the axis or star is at right angles to the flow directionof the neutralized solution (multiple addition points).

Particularly advantageously, two, three or four multiple addition pointsare arranged in succession.

For example, at least eight inlets Z₁ may be present, in which case fourinlets Z₁ in each case open in a cross shape into the ring line R, theat least 2 groups of four inlets Z₁ being arranged in succession andoffset relative to one another.

Moreover, at least one third inlet Z₃ may open into the ring line R,where at least one third inlet Z₃ means that reactant 3, for examplewater, is supplied via one or more inlets Z₃, and inlet Z₃ is beforeinlet Z₁ in flow direction and/or encloses inlet Z₁.

The distance between inlets Z₃ and Z₁ is preferably from 10 to 500%,more preferably from 50 to 300%, more preferably from 80 to 200%, of thesquare root of the ring line cross section Q.

The ring line R is preferably configured as a Venturi tube at least oneinlet Z₁ to Z₃.

The inlets more preferably open into a common Venturi tube.

The apparatus is preferably free of dead spaces; preferred materials arestainless steels, for example materials No. 1.4541 and No. 1.4571 to DIN17007.

The surfaces should have minimum roughness.

Dead spaces are sections of the apparatus in which the average residencetime is increased in the course of operation as intended.

EXAMPLES

The peak temperature of the following examples was calculated withFluent 6.0. The peak temperature is the highest temperature occurring inthe system. The program can be purchased, for example, via FluentDeutschland GmbH, Birkenweg 14a, D-64295 Darmstadt. Further supplierscan be found under www.fluent.com.

Examples 1 to 6

For Examples 1 to 6, an apparatus as described in FIG. 1 was assumed.For the calculation, the diameter of the ring line R was set at 20 cm,the diameter of the feeds Z₁ to Z₃ each at 5 cm, the mass flow rate inthe ring line R before feed Z₃ at 349 t/h, the temperature of the massflow rate in ring line R before feed Z₃ at 26° C., the distance betweenfeed Z₃ and feed Z₁ at 20 cm, the distance between feed Z₁ and feed Z₂at 20 cm, the mass flow rate of acrylic acid at 11.5 t/h, the mass flowrate of 50% by weight sodium hydroxide solution at 8.4 t/h and the massflow rate of water at 5 t/h.

Examples 1 to 3 demonstrate the influence of the metering sequence.

In Examples 4 to 6, sodium hydroxide solution and water are premixed. InExamples 5 and 6, the calculation was additionally supplemented bycooling of the sodium hydroxide solution/water mixture to thetemperature specified before entry into the ring line R.

The examples demonstrate how the peak temperature can be influenced viathe metering sequence.

Feed Temperature Peak Example Z₃ Z₁ Z₂ NaOH H₂O AA temperature 1 H₂O AANaOH 23° C. 23° C. 23° C. 64° C. 2 NaOH H₂O AA 23° C. 23° C. 23° C. 47°C. 3 AA H₂O NaOH 23° C. 23° C. 23° C. 64° C. 4 — NaOH/H₂O AA 23° C. 23°C. 23° C. 43° C. 5 — NaOH/H₂O AA 23° C.*) 23° C.*) 15° C. 38° C. 6 —NaOH/H₂O AA 15° C.*) 15° C.*) 15° C. 35° C. NaOH: 50% by weight sodiumhydroxide solution H₂O: water AA: acrylic acid *)Temperature of theNaOH/H₂O mixture

Examples 7 to 9

Examples 7 to 9 were calculated analogously to Examples 1 to 6.

For the calculation of Examples 7 to 9, it was assumed that an NaOH/H₂Omixture and acrylic acid are metered into a common Venturi tube. Thelength of the Venturi tube was set at 93.2 cm, the Venturi tubenarrowing to a diameter of 10 cm over a distance of 8.4 cm, retainingthe diameter of 10 cm over a distance of 27.6 cm and widening again to adiameter of 20 cm over a distance of 57 cm. The distance of the feed Z₁from the point where the Venturi tube has narrowed to 10 cm was assumedto be 5 cm, the distance of feed Z₂ from feed Z₁ to be 8 cm and thediameter of the two feeds Z₁ and Z₂ in each case to be 3.5 cm. The twofeeds Z₁ and Z₂ in each case are arranged opposite one another, theconnecting axis of the two feeds Z₁ being rotated by 90° relative to theconnecting axis of the two feeds Z₂.

In Examples 7 to 9, the influence of the mass flow rate in the ring lineR was additionally examined. In Examples 8 and 9, the mass flow rate inthe ring line R was therefore reduced by 50% and 90% respectively in thecalculation.

Temperature Exam- Mixing Feed NaOH/ Peak ple ratio Z₁ Z₂ H₂O AAtemperature 7  1:10 NaOH/H₂O AA 15° C. 15° C. 27° C. 8 1:5 NaOH/H₂O AA15° C. 15° C. 46° C. 9 1:1 NaOH/H₂O AA 15° C. 15° C. 69° C.

The calculations also demonstrate the advantages of the Venturi tubeover simple metering (comparison of Examples 6 and 7).

1. A neutralization process in which at least one ethylenicallyunsaturated carboxylic acid is neutralized at least partly with a base,which comprises carrying out the neutralization continuously and atemperature of the neutralized solution being less than 70° C.
 2. Theprocess according to claim 1, wherein the carboxylic acid and/or thebase is diluted with water.
 3. The process according to claim 1, whereinthe neutralized solution is cooled.
 4. The process according to claim 1,wherein the neutralized solution is recycled partly into theneutralization.
 5. The process according to claim 4, wherein between 25and 95% of the neutralized solution is recycled.
 6. The processaccording to claim 1, wherein the base is aqueous alkali.
 7. The processaccording to claim 4, wherein the recycled neutralized solution, in theneutralization, is admixed successively with the base and theethylenically unsaturated carboxylic acid.
 8. The process according toclaim 4, wherein the recycled neutralized solution, in theneutralization, is admixed successively with water, the base, and theethylenically unsaturated carboxylic acid.
 9. The process according toclaim 6, wherein the ethylenically unsaturated carboxylic acid has atemperature of from 15 to 25° C. and/or the aqueous alkali has an alkalicontent of less than 25% by weight and a temperature of from 15 to 30°C. or the aqueous alkali has an alkali content of at least 25% by weightand a temperature of from 30 to 50° C. and/or, if used, the water has atemperature of from 15 to 30° C.
 10. The process according to claim 1,wherein at least one metering point for the base, the ethylenicallyunsaturated carboxylic acid and, if desired, the water is designed as aVenturi tube.
 11. The process according to claim 6, wherein the aqueousalkali is a sodium hydroxide solution and/or the ethylenicallyunsaturated carboxylic acid is acrylic acid.
 12. A process for preparingwater-absorbing polymers, comprising a neutralization process accordingto claim
 1. 13. An apparatus for continuous neutralization, comprisingi) a ring line R, ii) at least one first inlet Z₁ into the ring line R,iii) at least one second inlet Z₂ into the ring line R, iv) at least oneheat exchanger W in the ring line R, the heat exchanger W being disposedbeyond the inlets Z₁ and Z₂ in flow direction, v) at least one outlet Afrom the ring line R, the outlet A being disposed beyond the heatexchanger W in flow direction, vi) a pump P, and vii) optionally, avessel B between the heat exchanger W and the outlet A.
 14. Theapparatus according to claim 13, wherein inlet Z₂ is disposed beyondinlet Z₁ in flow direction.
 15. The apparatus according to claim 13,wherein at least two inlets Z₁ and/or Z₂ are present.
 16. The apparatusaccording to claim 13, wherein at least eight inlets Z₁ are present, inwhich case four inlets Z₁ in each case open in a cross shape into thering line R, the at least 2 groups of four inlets Z₁ being arranged insuccession and offset relative to one another.
 17. The apparatusaccording to claim 13, wherein at least one third inlet Z₃ opens intothe ring line R, in which case inlet Z₃ is before inlet Z₁ in flowdirection and/or encloses inlet Z₁.
 18. The apparatus according to claim13, wherein the ring line R is designed as a Venturi tube at the openingof at least one inlet.
 19. The apparatus according to claim 18, whereinthe inlets open into a common Venturi tube.
 20. The apparatus accordingto claim 16, wherein at least one third inlet Z₃ opens into the ringline R, in which case inlet Z₃ is before inlet Z₁ in flow directionand/or encloses inlet Z₁.